Backfill for magnesium anodes

Abstract

A backfill composition for use with sacrificial magnesium anodes used in the cathodic protection of ferrous metal structures comprises an anion-releasing material capable of releasing fluoride, phosphate or zincate ions in water-soluble form and a magnesium-transporting adjuvant capable of transporting magnesium ion by ion exchange conduction. Suitable anion-releasing materials include calcium fluoride, cryolite, magnesium silicate and sodium silicofluoride, while suitable magnesium transporting adjuvants include bentonite clay, calcium sulfate, calcium carbonate, calcium hydroxide and magnesium silicate.

We claim: 
     
       1. A method of improving the efficiency of a magnesium anode comprising: contacting at least a part of the surface of said anode with a backfill composition comprising an anion-releasing material capable of releasing at least one of fluoride and silicate anions in water-soluble form and wherein said anion-releasing material comprises at least one of calcium fluoride, magnesium silicate, cryolite and sodium silicofluoride.   
     
     
       2. A method according to claim 1 wherein said anion-releasing material comprises calcium fluoride. 
     
     
       3. A method according to claim 1 wherein water is added to said backfill composition. 
     
     
       4. A method according to claim 1 wherein said backfill composition further comprises a magnesium-transporting adjuvant selected from the group consisting of bentonite clay, calcium sulfate, calcium carbonate and calcium hydroxide. 
     
     
       5. A method according to claim 4 wherein said magnesium transporting adjuvant comprises a mixture of bentonite clay, calcium sulfate and calcium hydroxide. 
     
     
       6. A method according to claim 4 wherein said backfill composition comprises from about 10 to about 75% by weight of said anion-releasing material and about 90 to about 25% by weight of said magnesium-transporting adjuvant. 
     
     
       7. A method according to claim 1 wherein said backfill composition further comprises a water-soluble salt to increase the conductivity of said composition. 
     
     
       8. A method according to claim 7 wherein said water-soluble salt comprises sodium sulfate. 
     
     
       9. A magnesium anode assembly for use in cathodic protection of ferrous metal structures, said anode assembly comprising: a magnesium anode; and   a backfill composition contacting at least part of the surface of said anode, said backfill composition comprising an anion-releasing material capable of releasing at least one of fluoride and silicate anions in water-soluble form and wherein said anion-releasing material comprises at least one of calcium fluoride, cryolite, magnesium silicate and sodium silicofluoride.   
     
     
       10. An assembly according to claim 9 wherein said anion-releasing material comprises calcium fluoride. 
     
     
       11. An assembly according to claim 9 wherein said backfill composition further comprises water. 
     
     
       12. An assembly according to claim 9 wherein said backfill composition further comprises a magnesium-transporting adjuvant selected from the group consisting of bentonite clay, calcium sulfate, calcium carbonate and calcium hydroxide. 
     
     
       13. An assembly according to claim 12 wherein said magnesium transporting adjuvant comprises a mixture of bentonite clay, calcium sulfate and calcium hydroxide. 
     
     
       14. An assembly according to claim 12 wherein said backfill composition comprises from about 10 to about 75% by weight of said anion-releasing material and about 90 to about 25% by weight of said magnesium-transporting adjuvant. 
     
     
       15. An assembly according to claim 9 wherein said backfill composition further comprises a water-soluble salt to increase the conductivity of said composition. 
     
     
       16. An assembly according to claim 15 wherein said water-soluble salt comprises sodium sulfate.